1. Field of the Invention
This invention relates to materials and methods for protecting metal surfaces, and more particularly to materials and methods for fabricating wear-resistant linings of composite barrels used in injection molding or extrusion.
2. Discussion
Injection molding and extrusion are widely used methods for shaping plastic articles. In single-screw extrusion, solid polymer granules are supplied at one end of a screw that rotates within a cylindrical bore of a temperature-controlled barrel. As the screw rotates, polymer granules advance along the barrel between flights of the screw and an inner wall that demarcates the cylindrical bore. Because of high temperatures and pressures, the polymer granules liquefy as they travel downstream within the barrel. Liquid polymer exits the barrel through a die, rapidly cools and solidifies. Depending on the application, the polymer is cut, rolled or undergoes a second forming operation.
Like extrusion, injection molding employs a screw that rotates in a temperature-controlled barrel to liquefy and consolidate polymer granules. However, in many injection molding machines, the screw also serves as an injection ram. During a plasticating step, the screw rotates and fills, with liquid polymer, a cavity in the barrel located adjacent to the screw tip. A small port, which is sealed with a plug of solidified polymer, connects the cavity with a mold. Pressure generated during the plasticating step is insufficient to dislodge the polymer plug; but during a subsequent injection step, the screw stops rotating and moves forward, forcing liquid polymer into the mold. A check ring located near the screw tip prevents liquid polymer from travelling backwards between flights of the screw.
During extrusion or injection molding, internal barrel pressure can reach 100 MPa or higher. To withstand these high pressures, barrels are typically made from carbon steel, alloy steel or stainless steel. The clearance between the rotating screw and barrel inner wall is small--about 10.sup.-2 cm--and therefore, polymer undergoing processing within the barrel exerts extremely high shearing forces on the bore surface. These high shearing forces, along with high barrel temperatures, corrosive polymer components and abrasive additives, such as TiO.sub.2, clay, silica, and carbon fiber, can rapidly erode conventional steel alloys. For this reason, many barrels have a wear-resistant layer or lining that is bonded to the bore surface. Barrels comprised of a steel outer housing and a wear-resistant lining are known as composite barrels or bimetallic barrels.
Composite barrel linings are typically applied using a process known as centrifugal casting. In this process, components of the lining--metal alloys and hard abrasion-resistant particles, for example--are placed in the cylindrical bore of the steel outer housing along with an amount of flux needed to minimize oxidation. Ends of the outer housing are capped to enclose the lining components. In some cases, the inner bore is evacuated and purged with argon gas. The barrel is placed in a high temperature furnace or induction coil and heated to a temperature sufficient to melt the metal alloy components of the lining. During heating, the barrel is often rotated slowly to evenly disperse the lining components. After the metals are melted, the barrel is removed from the high temperature furnace and is rapidly rotated to evenly distribute the lining components on the inner wall of the housing. As the outer housing cools, the metal solidifies and bonds to the inner wall of the outer housing, forming a wear-resistant layer. After the caps are removed, the bore of the composite barrel is honed to a desired diameter.
Although centrifugal casting is widely used to make wear-resistant linings for composite barrels, the method is expensive and its success depends on many factors. For example, the uniformity, morphology and hardness of the lining may depend on the form of the lining charge, the heating and cooling rates of the barrel, the amount of flux used, and whether the bore is evacuated or purged with argon gas during heating. Moreover, the use of argon dramatically increases the method's cost. Nitrogen gas, though much cheaper than argon, is unsuitable for centrifugal casting using known lining materials because its presence results in unacceptable lining quality, as evidenced by porosity, voids, oxidized particles, inclusions, etc. Because centrifugal casting depends on many factors, barrels are often re-spun or rejected, which increases the cost of the method.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.